Apparatus employing diode junction means for measuring subterranean temperatures

ABSTRACT

Apparatus for measuring temperatures and temperature anomalies along the longitudinal axis of a borehole penetrating subterranean formations characterized by a temperature sensor comprising a power supply and a temperature-compensated constant current regulator connected with at least one diode junction for flowing a constant current through the diode junction, the voltage drop across the diode junction varying linearly with temperatures over the range of 60*-350* F and linearly enough to be usable to temperatures as high as 400* F. In a specific disclosed embodiment, a unijunction transistor generates, in response to the voltage drops across the diode junction, frequency-type signals which are sent to the surface; an integrator is provided for integrating the signals to effect an indication of temperatures; and a differentiator is provided for differentiating the output from the integrator to effect a differentiated temperature indication to delineate anomalies. The temperatures and differentiated temperature indications are ordinarily recorded with respect to depth. Specific structures of the diode junction and of the temperature-compensated constant current regulator are also disclosed.

United States Patent Stout et al.

APPARATUS EMPLOYING DIODE JUNCTION MEANS FOR MEASURING SUBTERRANEANTEMPERATURES [451 June 27, 1972 Primary Examiner-Samuel FeinbergAssistant Examiner-N. Moskowitz Attorney-Wofibrd and F elsman [5 7]ABSTRACT Apparatus for measuring temperatures and temperature anomaliesalong the longitudinal axis of a borehole penetrating subterraneanformations characterized by a temperature sensor comprising a powersupply and a temperature-compensated constant current regulatorconnected with at least one diode junction for flowing a constantcurrent through the diode junction, the voltage drop across the diodejunction varying linearly with temperatures over the range of 60-350 Fand linearly enough to be usable to temperatures as high as 400 F. In aspecific disclosed embodiment, a unijunction transistor generates, inresponse to the voltage drops across the diode junction, frequency-typesignals which are sent to the surface; an integrator is provided forintegrating the signals to effect an indication of temperatures; and adifferentiator is provided for differentiating the output from theintegrator to effect a differentiated temperature indication todelineate anomalies. The temperatures and differentiated temperatureindications are ordinarily recorded with respect to depth. Specificstructures of the diode junction and of the temperaturecompensatedconstant current regulator are also disclosed.

4 Claims, 2 Drawing Figures F s5 FINDICATOR I J PULSE DIFFERENTIATORSHAPER AND AMPLIFIER 77 INTEGRATOR AND AMPLIFIER 8/ P'A'TENTEnJuuzv I97285 {INDICATOR l DIFFERENTIATOR AND AMPLIFIER INTEGRATOR AND AMPLIFIERTTOR/VEYS APPARATUS EMPLOYING DIODE JUNCTION MEANS FOR MEASURINGSUBTERRANEAN TEMPERATURES BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to measuring temperature in boreholespenetrating subterranean formations.

2. Description of the Prior Art It has long been recognized that aprofile of temperature along boreholes penetrating subterraneanformations could convey useful information to the petroleum engineer orgeologist. For example, expansion of gas from a gas zone or gas cap intoa wellbore effects a cooler than normal temperature. Such temperatureanomaly may effect a reversal in the thermal gradient at one or morepoints along the wellbore. Thief zones along the borehole may markabrupt changes in the temperature. In measuring solely the absolutetemperature along the borehole, the changes may be so small as to beoverlooked since they will be only a very small percentage of the total.If, however, a differential of the temperature is taken, the temperatureanomalies are delineated with great clarity. The problem in the priorart approaches has been to obtain a temperature sensor that was linearover the range of temperature encountered in most subterraneanformations; for example, temperatures of up to 350 F.

It is known in the prior art to employ thermocouples to measure downholetemperatures. With thermocouples the different metals form a junctionthat generates a spontaneous EMF which is indicative of the temperature.Thermocouples have been demonstrated to be disadvantageous because ofthe very small EMFs generated and the difficulties in translating thetemperature into useful information at the surface.

It is known in the prior art to use two temperature sensors; such as,thermistors; spaced a distance apart and their output compared. Thisapproach has been useful for several years in delineating temperatureanomalies. This approach was not altogether satisfactory since itdepended upon two essentially non-linear sensors spaced at differentpoints in the wellbore.

More recently, a single temperature sensor was employed in the downholetool and the output of the sensor at one location stored in a computerand the difference between it and a subsequent reading at another depthcomputed. This approach was expensive, elaborate, and complex; and alsosuffered when the temperature sensor was non-linear.

The prior art devices ordinarily employed thermistors in practicalembodiments although thermistors are notoriously non-linear. Sensistorswere tried. Although less non-linear than transistors, the sensistorshad positive temperature coefficients and were more difficult to employin borehole tools at the elevated temperatures encountered insubterranean formations, and they were still non-linear to anappreciable degree.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic representationof apparatus embodying the invention being used to log temperaturesalong a borehole penetrating subterranean formations.

FIG. 2 is a partial electrical schematic diagram showing the temperaturesensor and the temperature-compensated constant current regulator usefulin one embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS It is a primary object of thisinvention to provide method and apparatus for measuring temperaturesalong the longitudinal axis of a borehole penetrating subterraneanformations employing a temperature sensor that has a negativetemperature coefficient and is accordingly readily useful; and that islinear over the range of temperatures encountered in most subterraneanformations.

Other specific objects will be apparent from the following descriptivematter taken in conjunction with the drawings.

Referring to FIG. 1, downhole tool 11 is suspended in borehole l3penetrating subterranean formations 15 by cable 17. Cable 17 passes overdepth measuring sheave 19 which drives a depth function generating means21. Cable 17 is spooled from or onto cable drum 23, and has conductormeans electrically connecting elements in the downhole tool viaconventional slip rings and brushes 25 with the remainder of the surfaceequipment.

Downhole tool 11 comprises respective sections housing temperaturesensor 27, constant current regulator 29, amplifier means 31, signalgenerating means 33, transmitter, or output, means 35, power supplies37, collar locator and amplifier 39, and cable head 41.

Temperature sensor 27 comprises diode junction means having at aconstant current flow a substantially linearly varying voltage drop overthe range of temperatures of 60-350 F. While the diode junction meansmay comprise a single diode junction, it is preferable to employ aplurality of such diode junctions 43, FIG. 2, connected in series toincrease the sensitivity of the temperature sensor thereby effected and,consequently, reduce the requirements on the remainder of the electronicelements in the downhole tool. A particularly preferred temperaturesensor is formed by eight of the diode junctions created by seriallyconnecting two units of four diode junctions each. Diode junctionsformed in silicon diodes are excellent in providing the requisitelinearity between 60 and 350 F. In fact, they are linear enough to beuseful at temperatures as high as 400 F. Other diode junctions; such asthose formed in germanium or even in the compound semiconductormaterials; may be employed when they behave similarly. While thisinvention is not to be limited by theory, it appears that the diodejunction has available a greater number of electrons and holes forcarrying the constant current as the temperature increases.Consequently, there is a lower voltage drop across the junction atelevated temperatures. There is, in effect, a sort of an expansion ofthe junction also as the temperature increases. While different diodejunctions may have the desired linearly varying voltage characteristicsover certain ranges at different current levels, it has been found thatwith silicon diodes, a current level of about one-half milliampereaffords satisfactory results. The one-half milliampere is an excellentamount of current to break down the diode when it is forward biased, yetnot create other problems, such as overheating. Current levels may bevaried between a few hundredths of a milliampere to the temperature atwhich the diode begins to heat up; ordinarily, for example, about 60milliamperes.

Constant current regulator means 29 comprises a field effect transistor45, FIG. 2. Field effect transistor 45 has its gate lead 47 connectedwith one lead 49 which may be source or drain depending on polarity ofthe interconnection. Field effect transistor 45 has its other lead 51connected with respective diode junction means 43. To maintain the flowof current constant regardless of change in temperature as the downholetool 11 traverses borehole 13, a series connected resistor meanstypified by resistor 53 and a diode means typified by diode 55 areemployed in the circuit connected with intermediate lead 49 and thejuncture of gate lead 47 and power source 57. The resistor 53 and diode55 may be reversed and obtain the same result. Power source 57 isindicated as a battery although it may be a DC voltage tapped from anyappropriate source.

Although the embodiment illustrated in FIG. 2 forms a simple anddependable constant current regulator, any other constant currentregulator means that will maintain the current constant may be employed.In any event, the constant current regulator means and the power supplymeans are serially connected with the diode junction means formaintaining a constant current flowing through the diode junction means.

An amplifier means 31 in the operational amplifier section of thedownhole tool is connected with the diode junction means and adapted forgenerating an output signal that is a function of the voltage dropacross the diode junction means.

As illustrated in FIG. 2, conductor 59 connects the amplifier means withthe juncture of the diode junction means and the constant currentregulator means. The amplifier means generates an output signal that isa function of the voltage present at this juncture. The amplifier meansmay be any amplifier capable of generating an output signal that isuniquely representative of the voltage present at the juncture of thediode junction means and the constant current regulator means. Forexample, it has been found that an operational amplifier capable ofeffecting a voltage between a positive volts and a negative 15 voltsupon the change of the voltage on conductor 59 of from about a negative5.6 volts down to about a negative 3 volts is satisfactory. The termoperational amplifier is a term of art and adequately describes theamplifier means for effecting the desired results. Suitable operationalamplifiers are commercially available. An operational amplifier that hasbeen found satisfactory is the Philbrick T-S 2. It effects outputvoltages beginning at about plus 7.5 volts (v) at a voltage input ofminus 5.6 v on conductor 59; growing more negative at a ratio of about9:1; and finally effecting about minus ll v output at a voltage input ofabout minus 3 v on conductor 59. With a sufficient number of diodejunctions in the diode junction means, the amplifier means may beomitted. For example, with about 80 diode junctions serially connectedin the diode junction means, the amplifier means can be omitted, and thetool remain operable with only a minor modification of the signalgenerating means, as described hereinafter.

The signal generating means 33 in the signal generating means section ofthe downhole tool is connected with the amplifier means for generatingsignals that are a function of the output signal from the amplifiermeans. Specifically, a unijunction transistor may be employed tooscillate at a frequency that increases as the voltage output signalgrows more negative. Thus, it will be seen that the frequency-typesignal from the unijunction transistor is a function of the temperaturebeing measured by temperature sensor 27. For example, one unijunctiontransistor found useful oscillates at an output of about five cycles persecond per degree F. This increasing frequency coupled with the initialthreshold of oscillation produces about 2,000 cycles per second at atemperature of about 300 F. The unijunction transistor employed as thesignal generating means should preferably be linear over the ranges ofthe output signal. It has been found that a D5K1 or a 2N492 typeunijunction transistor will operate satisfactorily. These unijunctiontransistors are commercially available. While theunijunction transistorhas been described as the signal generating means and converts theoutput signal of the amplifier means into a frequency-type signal, anyother signal generating means may be employed if it will effect anoutput signal that can be transmitted to the surface and be uniquelyrelated to a temperature measured in the borehole. If a large number ofdiode junctions are employed in the diode junction means, the amplifiermeans is omitted, and the same relationship of frequency and temperatureare employed, then the polarity impressed across the diode junctionmeans is reversed. On the other hand, the polarity may be preserved andan inverse relationship of frequency and temperature employed such thatthe frequency decreases as the temperature increases. Where theamplifier means is omitted and the signal generating means is connecteddirectly to the diode junction means, it may be advisable to alter thevoltage level of operation of the signal generating means. In theexample given hereinbefore, employing about 80 diode junctions in thediode junction means, the upper voltage of operation of the unijunctiontransistor, comprising the signal generating means, might be about apositive 50 volts and grow more negative with increasing temperature toabout a positive 30 volts, the voltage base being maintained by suitablemeans such as Zener diodes. The unijunction transistor would accordinglysee about the same voltage change it presently sees by way of theoperational amplifier.

The transmitter means 35 in the output section of downhole tool 11 isconnected with the signal generating means for transmitting the signalto the above-ground equipment. It has been found that a satisfactorytransmitter means comprises a transformer and two transistors. Theresulting transmitter means will satisfactorily transmit thefrequency-type signals to the above-ground equipment.

In the above-ground equipment, converting means is connected with thetransmitter means in the downhole tool for effecting an analogindication of the temperature encountered in the wellbore in response tothe received signals. As illustrated in FIG. 1, the frequency-typesignals are sent through coupling capacitor 61 and via conductor 63 topulse shaper 65 for accurate pulse control and, thence, via conductor 67to integrator and amplifier 69. In the integrator, the frequencytypesignals are integrated and amplified to efiect an analog indication ofthe temperature in the borehole in response to the signals. The analogindication is sent over conductor 71 to recorder 73 on which a chart isbeing moved in response to depth signals from the depth functiongenerating means 21 via conductor 75. If desired, an indicator can beconnected with conductor 77 to effect a direct indication of thetemperature being logged without having to advert to the chart onrecorder 73. The analog indication of temperature from integrator andamplifier 69 is also sent via conductor 77 to differentiator andamplifier 79. The differentiated and amplified analog signal fromdifferentiator and amplifier 79 is also sent to recorder 73 viaconductor 81'.

Conventional integrating circuits and conventional differentiatingcircuits can be employed for integrating the frequency-type signals andfor differentiating the resulting analog signal. As indicated before,the analog signal afiords an indicia of the temperature actuallymeasured at a given depth in the borehole. Accordingly, it changesrelatively little, and minute changes might escape detection. On theother hand, a differential of the analog signal indicates a change inthe temperature gradient and is very useful in delineating temperatureanomalies, although it is not of value for determining the actual, orabsolute temperature measured in the borehole. Expressedotherwise, theanalog signal from integrator and amplifier 69 affords a good indicationof the actual temperature being measured whereas the differentiatedsignal is too unstable to be very useful in this respect, but is veryuseful in pointing up changes in the temperature gradient and thusdelineating temperature anomalies.

While, as indicated hereinbefore, batteries can be employed in thedownhole tool, it has been found preferable to supply power at thesurface and generate power of the proper polarities for the respectivepower sources downhole. As illustrated in FIG. 1, a DC power source 83is connected downstream of coupling capacitor 61 to facilitate supplyingDC power downhole. It has been found that a positive volts can besupplied and enable obtaining the requisite AC and Dc power in thedownhole tool via appropriate power supply means 37 in power supplysection.

it has been found, for example, that a minus 24 volts DC may begenerated by DC--DC converter in the tool and it affords a satisfactorypower source to which to connect the constant current regulator means.This enables satisfactorily controlling the current through the diodejunction means at a desired current level of about one-half milliampere.

In the power supply section, the output from the DC-DC converter may berectified or respectively tapped and rectified to effect the requisiteplus or minus 15 volts for the opera tional amplifier.

In operation, tool 11 is traversed along the longitudinal axis ofborehole 13. Temperature sensor 27 will effect a voltage on conductor59. As indicated, the voltage may start out about a minus 5.6 volts for8 diode junctions in series at surface temperatures and decreaselinearly with increasing temperature to a voltage in the range of abouta minus 3 volts at temperatures of about 350 F. The operationalamplifier amplifies the signal to form an output signal that is afunction of the voltage and consequently of the temperature. Responsiveto the output signal from the operational amplifier, the unijunctiontransistor oscillates at a given frequency for each magnitude outputsignal. The frequency-type signal, consisting of at least pulses of agiven polarity, are transmitted uphole by a transmitter means 35 in theoutput section. The collar locator and amplifier in collar locatorsection 39 may be employed, if desired, to ensure greater accuracy inmeasuring the depth within the borehole and as a supplemental check onthe depth measuring means. In any event, the frequency-type signal istransmitted over a single conductor in cable 17, the armor of whichserves as ground to slip rings and bushes 25 on cable drum 23.

As indicated before, power is supplied from power supply 83 throughcable 17 to the power section where it is appropriately tapped togenerate the requisite voltages for use in downhole tool 1 1.

The frequency-type signal is sent by way of coupling capacitor 61, whichblocks the direct current power from power source 83', to pulse shaper65. Pulse shaper 65 effects uniform pulses'which are integrated byintegrator and amplifier 69. The resulting analog signal is sent torecorder 73 to record with respect to depth the temperature measured inthe borehole. The analog signal is also differentiated by differentiatorand amplifier 79 and the resulting differentiated analog signal recordedvia a pen on recorder 73 to delineate temperature anomalies, or changesin the temperature gradient.

Thus, it can be seen that the invention provides a highly accuratetemperature sensor that has a linear output over the range oftemperatures normally encountered in boreholes penetrating subterraneanformations, yet has a negative temperature coefficient so it is readilyemployed in borehole apparatus; and, consequently, obviates thedisadvantages of the prior art approaches to measuring temperature in aborehole.

Although the invention has been described with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that numerous changes in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention.

What is claimed is:

1. In apparatus for measuring temperature and change in temperature in aborehole penetrating subterranean formations by employing abovegroundequipment; a downhole tool suspended on a cable within said borehole formovement along the longitudinal axis of said borehole; and depthmeasuring means for determining the depth of said tool in said borehole,the improvement comprising:

a. diode junction means for measuring temperature said diode junctionmeans being disposed in said downhole tool and having a substantiallylinearly varying voltage drop at a constant current within thetemperature range of 60-350 F;

b. a power supply means and a temperature-compensated constant currentregulator means for maintaining a constant current flowing through saiddiode junction means regardless of variations in temperature along saidborehole, said power supply means and said temperaturecompensatedconstant current regulator means being serially connected in the forwardbiased direction with said diode junction means; saidtemperature-compensated constant current regulator means comprising afield efiect transistor serially connected via one lead with said diodejunction means and via the other lead with a resistor means and a diodemeans for temperature compensation; the gate of said field effecttransistor being connected with said other lead at the juncture of saidpower source and said serially connected diode'means and resistor means;

c. signal generating means for generating signals that are a function ofthe voltage present at the juncture of said diode junctionrneans andsaid constant current regulator means, said signal generating meansbeing connected with said diode junction means;

d. transmitter means for transmitting said signals to the above-groundequipment, said transmitter means being connected with said signalgenerating means and with conductor means in said cable for effecting anelectrically continuous path; 4

e. converting means for effecting an analog indication of saidtemperature in response to said signals, said converting means beingdisposed in said above-ground equipment and connected via said conductormeans with said transmitter means;

f. differentiator means for differentiating said analog indication oftemperature to efiect an indication of temperature anomalies along thelongitudinal axis of said borehole, said differentiator means beingconnected with said converting means; and

g. recorder means for recording with respect to depth said analogindication of temperature and said indication of temperature anomalies,said recorder means being connected with said depth measuring means,with said converting means and with said difierentiator means.

2. The apparatus of claim 1 wherein said diode junction means comprise aplurality of diode junctions employed in series to increase sensitivityof the temperature sensor thereby effected.

3, The apparatus of claim 1 wherein an amplifier means is seriallyconnected with said diode junction means and with said signal generatingmeans for generating and supplying to said signal generating means anoutput signal that is a function of the voltage present at said junctureof said diode junction means and said constant current regulator means.

4. The apparatus of claim 3 wherein said diode junction means comprise aplurality of diode junctions employed in series to increase sensitivityof the temperature sensor thereby effected.

1. In apparatus for measuring temperature and change in temperature in aborehole penetrating subterranean formations by employing abovegroundequipment; a downhole tool suspended on a cable within said borehole formovement along the longitudinal axis of said borehole; and depthmeasuring means for determining the depth of said tool in said borehole,the improvement comprising: a. diode junction means for measuringtemperature, said diode junction means being disposed in said downholetool and having a substantially linearly varying voltage drop at aconstant current within the temperature range of 60*-350* F; b. a powersupply means and a temperature-compensated constant current regulatormeans for maintaining a constant current flowing through said diodejunction means regardless of variations in temperature along saidborehole, said power supply means and said temperature-compensatedconstant current regulator means being serially connected in the forwardbiased direction with said diode junction means; saidtemperaturecompensated constant current regulator means comprising afield effect transistor serially connected via one lead with said diodejunction means and via the other lead with a resistor means and a diodemeans for temperature compensation; the gate of said field effecttransistor being connected with said other lead at the juncture of saidpower source and said serially connected diode means and resistor means;c. signal generating means for generating signals that are a function ofthe voltage present at the juncture of said diode junction means andsaid constant current regulator means, said signal generating meansbeing connected with said diode junction means; d. transmitter means fortransmitting said signals to the aboveground equipment, said transmittermeans being connected with said signal generating means and withconductor means in said cable for effecting an electrically continuouspath; e. converting means for effecting an analog indication of saidtemperature in response to said signals, said converting means beingdisposed in said above-ground equipment and connected via said conductormeans with said transmitter means; f. differentiator means fordifferentiating said analog indication of temperature to effect anindication of temperature anomalies along the longitudinal axis of saidborehole, said differentiator means being connected with said convertingmeans; and g. recorder means for recording with respect to depth saidanalog indication of temperature and said indication of temperatureanomalies, said recorder means being connected with said depth measuringmeans, with said converting means and with said differentiator means. 2.The apparatus of claim 1 wherein said diode junction means comprise aplurality of diode junctions employed in series to increase sensitivityof the temperature sensor thereby effected.
 3. The apparatus of claim 1wherein an amplifier means is serially connected with said diodejunCtion means and with said signal generating means for generating andsupplying to said signal generating means an output signal that is afunction of the voltage present at said juncture of said diode junctionmeans and said constant current regulator means.
 4. The apparatus ofclaim 3 wherein said diode junction means comprise a plurality of diodejunctions employed in series to increase sensitivity of the temperaturesensor thereby effected.